"Cellulose Insulation Guide: Recycled Paper Fiber, Blown-In, Wet-Spray, Dense-Pack Methods, and R-Values"

Cellulose insulation is a loose-fill fibrous insulation made primarily from recycled paper, predominantly post-consumer newsprint. It is one of the oldest forms of insulation still in widespread use and, measured by embodied energy and recycled content, the most environmentally benign of all common insulation materials. The typical composition is 85 percent recycled paper fiber and 15 percent fire retardant chemicals, a formulation that balances material performance with cost and sustainability.

Composition and Fire Retardant Chemistry

The fire retardant used in cellulose insulation is a borate-based mixture of boric acid and sodium borate (borax). These chemicals serve multiple functions beyond fire resistance: they act as an insecticide, deterring pests such as cockroaches, ants, and termites from nesting in the insulation; they function as a fungicide, preventing mold growth even in damp conditions; and they inhibit corrosion of metal fasteners and electrical components that contact the insulation. The borates are water-soluble, which means they can leach out if the insulation is exposed to persistent water infiltration, but under normal building conditions they remain stable and effective for the life of the structure.

When exposed to fire, cellulose insulation forms a surface char layer that insulates the underlying material and slows flame propagation. This charring behaviour, combined with the borate chemistry, allows cellulose to achieve a Class 1 fire rating under ASTM E84.

Installation Methods

Cellulose is installed using three distinct methods, each suited to different building assembly types. Blown-in dry cellulose is pneumatically conveyed through a hose and distributed into open attic cavities. It achieves a density of 1.5 to 2.0 lb per cubic foot and an R-value of R-3.2 to R-3.8 per inch. This is the most common residential application and is particularly effective in attics with irregular joist spacing or obstructions.

Wet-spray cellulose is applied in open wall cavities using a water spray nozzle at the hose end that activates a small percentage of adhesive mixed into the product. The water allows the cellulose to adhere to vertical surfaces and fill the cavity without netting, then the cavity is screeded flush with the stud faces. Once dry, typically within 24 to 48 hours depending on climate, the material achieves its design R-value.

Dense-pack cellulose is blown into enclosed wall cavities through access holes at a higher density of 3.0 to 3.5 lb per cubic foot. This method is used for retrofitting existing walls and provides a significant air-sealing benefit because the densely packed fibers resist air movement through the cavity. Dense-pack cellulose is widely regarded as one of the most effective retrofit insulation strategies for older homes with uninsulated wall cavities.

Thermal Performance

Cellulose insulation achieves R-3.2 to R-3.8 per inch, with higher densities yielding higher R-values. Unlike fiberglass, which loses R-value significantly at very low temperatures due to convection within the batt, cellulose maintains its thermal performance across a wide temperature range. The dense fiber structure suppresses internal air movement, so the R-value remains stable even at extreme cold temperatures, making it well-suited for attic insulation in cold climates.

Embodied Energy and Sustainability

Cellulose insulation has the lowest embodied energy of any common insulation type. Because approximately 85 percent of the material is recycled paper that requires only shredding and fiberizing rather than extraction or chemical synthesis, the manufacturing energy is a fraction of that required for mineral wool, fiberglass, or foam insulation. Life-cycle assessment studies consistently rank cellulose at or near the top among insulation products for environmental performance, both in terms of manufacturing impact and operational energy savings.

Major Brands

GreenFiber, a joint venture initially formed by several large paper companies, is the largest cellulose insulation manufacturer in North America. Applegate Insulation operates multiple plants across the United States and supplies both residential and commercial markets. Cocoon, a brand produced by US GreenFiber, is widely distributed through home improvement retailers. All three brands offer dry loose-fill, wet-spray, and dense-pack product formulations with similar borate fire retardant chemistry.